1. Field of The Invention
The invention relates to communication devices which are assigned to each other in pairs, one unit sending out an identification code for establishing a link to the other unit over one of n channels and the other unit searching for the channel transporting this identification code while a number of m smaller than n channels between the two units are determined as priority channels.
2. Related Art
Communication devices assigned to each other in pairs are used, for example, for cordless telephones where a base station (unit) and a mobile station (unit) form a pair of communication stations. The combination of the base station and mobile station to one pair of units is effected by means of an individual identification code jointly issued to the two units. This identification code is, for example, a binary data word which is already stored in read-only memories of the two units when manufactured. This assures that each identification code assigned to a pair of units is not assigned a second time to another unit. The power of the mobile unit is supplied by accumulators which are charged by the base unit when the mobile unit rests on it. The base unit sends out its identification code over an idle radio channel when a call arrives from a telephone subscriber. In order to take an arriving call the mobile unit thus has to carry out a constant check of the radio channels assigned thereto to establish whether its own identification code is transmitted over a radio channel.
The time elapsing between the transmission of the unit's identification code and its recognition depends among other things on the number of radio channels to be tested. To reduce this time interval European Patent Application No. EP 0 258 739 discloses that the two radio stations of each pair have m radio channels in common and store them in RAM's. Radio channels recognized as idle, when the channels are determined, are selected to act as common radio channels. During the test cycle the mobile unit only searches the common radio channels. Consequently, the period of time necessary for testing the common channels is shorter than the period of time necessary for testing all channels.
If, by chance or malfunction, all common channels are busy establishing a link between the base unit and the mobile unit will no longer be possible.